Induction heating is performed for a workpiece where areas to be heated are established so as to extend in one direction. Heat treatment equipment such as follows has been proposed to heat a large workpiece in a shape of a ring, for example: heating coils are disposed so as to face a portion of areas to be heated extending in one direction on a workpiece, and by rotating the workpiece, for example, the entire length of the areas to be heated of this workpiece is heated.
The following Patent Reference 1 discloses high-frequency heat treatment equipment capable of heating a ring-shaped workpiece such as a large bearing ring. With this technique, a plurality of horseshoe-shaped heating coils are disposed at places along the circumferential direction of the ring-shaped workpiece, the induction heating is performed by the plurality of heating coils while the ring-shaped workpiece is being rotated, and after the heating, a cooling liquid is discharged from each of the horseshoe-shaped heating coils to rapidly cool the workpiece for heat treatment.
The following Patent Reference 2 discloses a device for performing induction heat treatment of the entire body of a ring-shaped piece. With this technique, positioning of the outside surface and inside surface of the ring-shaped piece is performed using a positioning roller, heating coils are disposed partially on the inner periphery, outer periphery, side face, etc. of the ring-shaped piece, the induction heating is performed while the ring-shaped piece is being rotated, and then rapid cooling is performed as heat treatment.
An induction heating device for applying electric power to heating coils as described below is generally used. Namely, such an induction heating device includes: an inverter for converting commercial power into direct current once and then converting it into alternating current having a predetermined frequency; a transformer including a primary winding connected to the inverter and a secondary winding; matching units connected in parallel to the primary winding of the transformer; and a heating coil connected to the secondary winding of the transformer. The heating coil and a workpiece are inductively coupled to ensure matching between the inverter and the heating coil.
The induction heating device disclosed in Patent Reference 3 is configured by connecting a capacitor and a matching transformer to a self-controlled thyristor inverter as a power supply, connecting a group of taps for selecting proper voltage and proper frequency at the time of increasing-temperature heating of an object to be heated, connecting another group of taps for selecting proper voltage and proper frequency at the time of constant-temperature heating of the object to be heated, and connecting each group of taps to a heating coil via a selector switch.